Blood Oxygenators
In known blood oxygenators, hollow fibers are used as a means to bring blood into contact with oxygen and provide a means for removal of carbon dioxide from the blood. The fibers are typically made of a homogeneous membrane of gas-permeable material such as silicone or of hollow fibers made of a microporous membrane of hydrophobic polymeric material such as polyolefins.
There are two types of hollow fiber blood oxygenators: the inside perfusion type in which blood is passed through the bores of the hollow fibers while oxygen is passed on the outside of the hollow fibers, and the outside perfusion type. Blood oxygenators of the outside perfusion type pass oxygen through the bores of the hollow fibers while blood is flowed past the outside of the hollow fibers.
Examples of inside perfusion type hollow-fiber oxygenators are disclosed in U.S. Pat. Nos. 4,239,729 and 4,749,551.
In blood oxygenators of the outside perfusion type the oxygen can be distributed uniformly through the spaces between adjacent fibers and the blood can be expected to move with better mixing. However, outside perfusion has had the disadvantage of being subject to less than the desired oxygenation of the blood because of region channeling of the blood as it passes the outsides of the hollow fibers. Blood-side convective mixing is essential for efficient gas transfer in blood oxygenators. Without such mixing, sharply defined boundary layers of fully oxygenated blood develop near the exchange surfaces and the fluxes of oxygen and carbon dioxide tend to be low. Low transport efficiency results in bulky devices with undesirable high blood priming volumes.
Outside perfusion type blood oxygenators are known in which the hollow fibers are in perpendicular orientation to the direction of blood flow so as to produce more mixing of the blood as the blood flows than inside perfusion constructions. This arrangement can bring about an improvement in oxygenation rate. However, if the number of fibers used in such a blood oxygenator is large (as is desirable) and/or the flow rate of blood is increased in order to treat large volumes of blood, problems arise. For example, unacceptable pressure drop of the blood between inlet and outlets and/or channeling of the blood between groups of fibers may occur. By channeling it is to be understood that a significant flow of blood takes place through relatively large area voids between fibers so that there is little or no mixing. As the rate of oxygen transfer primarily takes place in a thin boundary layer adjacent the hollow fibers, the effectiveness of desired oxygenation is reduced.
Examples of blood oxygenators of the outside perfusion type are disclosed in copending application PCT/US89/00146 filed Jan. 13, 1989; WO 89/00864; and U.S. Pat. Nos. 3,794,468; 4,352,736; 4,622,206; 4,659,549; 4,639,353; 4,620,965; 4,791,054; and 4,808,378, all incorporated herein by reference.